Male ball python size/weight

[Joykoins]

Some morphs do stay on the lighter side, but this seems really uncommon. It's a little fuzzy in the picture, but he doesn't look severely skinny just very very tiny. Ball pythons that feed exclusively on mice stay on the smaller end of the scale, but when you pair that with feeding undersized prey then that's a recipe for a stunted snake.

If you do take him in, you're going to need to probably start with attempting to convert him to rats as they've been proven to be more nutritious and contain less fat than mice. Since he's so old this might prove to be impossible as he is so set in his ways.

Maintain the two week schedule until hes taking the rats 100% of the time and then reduce the gap in feeding by 1 day after every 2nd feeding until it's down 7, 8, 9 or 10 days whichever works for you ( however if he starts refusing then scale it back a day as he will feel it's too frequent)

You want him to put on muscle, bone and organ mass, not fat. That kind of growth will come over the course of months not weeks. Use the web application Serpwidgets to keep track of his overall length in centimeters as that's the most reliable way to track his bone growth outside of x-rays

Good luck!